In a manufacturing process of various semiconductor devices, a silicon nitride film is formed, as a gate insulating film of a transistor, on a semiconductor substrate. As for a method for forming a silicon nitride film, in addition to a method for depositing a silicon oxide film by CVD (Chemical Vapor Deposition), there is suggested a method for forming a silicon oxynitride film by introducing nitrogen into a silicon oxide film by plasma processing (e.g., Japanese Patent Laid-open Publication No. 2001-274148).
Further, as semiconductor devices are miniaturized, a gate insulating film becomes thinner, and it is required to form a gate insulating film having a film thickness of a few nm. Therefore, there is examined a possibility to form a silicon nitride film by nitriding silicon directly.
As for a method for forming a gate insulating film by introducing nitrogen into a silicon substrate directly, there is suggested a method for forming an insulating film including: a nitride film forming step of forming a first nitride film on a semiconductor substrate; an oxide layer forming step of forming a first oxide layer between the semiconductor substrate and the nitride film and also forming a second oxide layer on the nitride film; and an oxide layer nitriding step of forming a second nitride film or an oxynitride film on the first nitride film by nitriding the second oxide film (e.g., Japanese Patent Laid-open Publication No. 2005-93865 (JP2005-93865A)). This method has a purpose of obtaining a uniform film thickness of the formed gate insulating film and reducing an equivalent oxide thickness (EOT).
In the method disclosed in JP2005-93865A, the silicon nitride film is formed by nitriding the silicon substrate directly. Then, the silicon oxide film, the first silicon nitride film and the second silicon nitride film (or silicon oxynitride film) are formed on the interface of the silicon substrate by performing oxidation and nitriding. However, the gate insulating film formed by this method is disadvantageous in that interface states and fixed charges thereof change a threshold voltage and increase a flat band voltage (Vfb), thereby adversely affecting the mobility of electrons and holes in a transistor. That is, in the technique disclosed in JP2005-93865A, it is difficult to form a high-quality gate insulating film which ensures good electrical characteristics of a transistor.